The invention is based on a high-voltage bushing as claimed in the precharacterizing clause of patent claim 1.
In the precharacterizing clause of patent claim 1, the invention refers to a prior art as described in EP-A2-0 388 779. The already known bushing is in the form of a cable plug connection and has a varistor arrangement in the form of a tube, as well as an electrical conductor, which is guided along the tube axis and is provided with external insulation, in a high-voltage cable, a field-controlling insulating body arranged between the external insulation and the inner surface of the tube, and an external weather-resistant insulator surrounding the varistor arrangement. An electrical connection, which is connected to a metallic, grounded protective sheath of the cable, is electrically conductively connected via spring elements to the lower end of the varistor arrangement. The upper end of the varistor arrangement is electrically conductively connected to an electrical connection, which is connected to the electrical conductor and to a connecting conductor located outdoors. A threaded nut which is supported on the upper electrical connection is rotated to prestress the connecting conductor and the electrical conductor that is connected by a force fit to it, hence forming the contact force in the varistor arrangement as well as between the lower and upper end of the varistor arrangement and the two electrical connections.
This bushing is distinguished in that, in a confined space, it not only allows the cable electrical conductor to be securely passed out of the grounded cable sheath but also allows overvoltages, which can be brought about via the connecting conductor that is connected, for example, to an overhead line, to be limited.
Metal-oxide surge arresters, which are insulated by polymer plastics, for explosionproof operation and based on the design principles of composite insulators, layer composites and direct casting are specified in the publication by Walter Schmidt, Metalloxidxe2x80x94ein fast idealer xc3x9cberspannungsableiter [Metal oxidexe2x80x94a virtually ideal surge arrester], Swiss Bulletin SEB/VSE July/1998, pages 13-20. Complete integration of the metal-oxide resistors in the polymer insulation structure improves the insulation level and the system reliability and reduces costs. Integration of the surge arresters with other electrical components, such as cable end terminations or bushings, for example for transformers, allows a further improvement in the protective function to be expected. No further details are given of the configuration of this integration.
CH 659 550 A5 specifies voltage-limiting conductive bushings with varistors as overvoltage limiters, in which a number of varistors in the form of annular disks, and preferably made of zinc oxide, are electrically connected in series, and the cylindrical electrical conductor is passed through their central opening. The mutually facing contact surfaces of the varistor disks can be separated by a thin, electrically conductive intermediate layer, for example composed of solder or diffusion welding agents.
It is known from the publication by Jeffry P. Mackevich and John W. Hoffmann, Insulation Enhancement with Heat-Shrinkable Components Part III: Shield Power Cable, IEEE Electrical Insulation Magazine, July/August 1991, Vol. 7, No. 4, pages 31-40, for electrical cables to be electrically shielded on their dielectric cable insulation layer with a specific electrical resistance of 104 xcexa9xc2x7cm by means of a semiconductive layer, referred to as a semicon layer, which may be applied by extrusion or being wound on. Cable connections and end terminations are electrically insulated externally and are electrically shielded in places, in order to minimize field peaks and to ensure protection against direct contact.
The invention, as it is defined in the patent claims, is based on the object of further developing a bushing of the type mentioned initially, such that it can be tested, installed and maintained in a simple manner.
In the bushing according to the invention, a surge arrester which is integrated in the bushing is formed by using an insulating part to stress the two electrical connections and the varistor arrangement. The bushing can thus be produced without using any electrical conductor inserted in a cable or in an electrical apparatus, for example a transformer. Since, at the same time, a supporting element which is provided with a guide surface is formed in the bushing insulator, for a plug-in contact (which is connected to the electrical conductor) of a plug connection to a cable or to an electrical apparatus, the bushing insulator can be prefabricated completely and can be fitted at the installation location quickly and easily by plugging it to a housing bushing of an electrical apparatus, such as a transformer, or to a cable.
The guide surface is in the form of an external or internal cone, for connection to a conical end of an apparatus bushing or of a cable. For connection to a cylindrical end of an apparatus bushing or of a cable, for example a cable sleeve, the guide surface is in the form of an outer surface of a cylinder or an inner surface of a hollow cylinder. The interaction of the guide surface with a corresponding guide surface on the apparatus bushing or the cable allows a mechanically strong and dielectrically high-quality plug connection to be produced in a simple and reliable manner.
In one particularly advantageous embodiment of the invention, the electrical connection which can be connected to high voltage is arranged detachably, and can be replaced by an insulating test cap. When the bushing is being manufactured, the test cap composed of insulating material can then be installed first, instead of this electrical connection. This considerably reduces the voltage drop in the surge arrester. The bushing and the apparatus connected to it, or the system connected to it, can now be loaded with high-voltage test pulses, which would otherwise destroy the surge arrester.
In a further advantageous embodiment of the bushing according to the invention, an overcurrent protective device 20 is installed in a section of the electrical conductor surrounded by the varistor arrangement. Without occupying any significant additional amount of space, this bushing limits not only voltages but also currents at the same time.
Effective protection of the electrical conductor, or insulation of the electrical conductor, in the event of the varistor arrangement being overloaded is provided if the clamping apparatus is tubular, and is arranged between the electrical conductor and the varistor arrangement.
Particularly compact bushings with a good dielectric behavior are achieved if the varistor arrangement has hollow varistors with an internal profile matched to the shape of the electrical conductor, and/or with an oval or rectangular external profile, and/or if the varistor arrangement is hollow and axially symmetrical, and has a conical or bottle-shaped contour in the direction of the electrical conductor.
The varistors are expediently split in the direction of the electrical conductor, since the varistors in the varistor arrangement can then not only be produced particularly easily but, furthermore, this also ensures that the bushing can be installed and maintained particularly easily.
The dielectric behavior of the bushing can be additionally improved if the bushing insulator on the supporting body has a voltage-linearizing effect.